Anyone heard of it?

Try Google.

The move order seems to be 1. d4 c5 2. d5 f5.

yeah, i've seen one game before, i was interested in learning the ideas behind it

As I remember, it was named after the flats that GM Jon Levitt lived in when he played it. There's apparently a page on his website about it at http://www.jlevitt.dircon.co.uk/clarendon.htm though it seems to be down at the moment. I believe he also made a DVD about it, though I've never seen it.

What the ideas are, I'm not sure. I've never played it myself, though I have faced it. About all I know from the white side is to get e4 in quickly. 

Thanks, some good reading

Mamedjarov played it recently against Seirawan, and lost. Yasser played the move 2...f5 is supposed to prevent: 3.e4!

http://www.chessgames.com/perl/chessgame?gid=1628449

Thanks for your game and your annotations. It's certainly playable.

My database shows a dozen or so games played this year, although i haven't really spent any time looking at them.

I was of the understanding that black's idea was to have the pawn chains f5, g6, h7; c5, d6, e7; and a6, b7, with the idea that white would have extreme difficulty in penetrating. I see the idea of white playing e4, a logical idea taken from the Staunton Gambit in the Dutch defence

There are a number of ways White gets in e4 by force, not the least of which is simply on move 3.  The point is that the onus is on White to prove some advantage or an attack from it.  Particularly in lines where White plays c4, White must also concern himself with the a6-b5 push, prepared by Na6/c7, Rb8, Bd7, and Qe8.  When c4 is not played, at least in my opinion, Black does well to organize his Rooks on f8 and e8 and play for e5 himself, when Black gets sufficient activity and an unbalanced game.

I hope I haven't painted too rosy a picture for Black, because his game is certainly not easy and innocuous-looking missteps, such as in the Gormally game, can prove quite inconvenient.  Move order is important as it is dangerous to play d6 before Na6 because of White's Ng5/Ne6, but it's also bad to delay it too long, when White may simply play Pd6 himself and gain a nice space advantage.